Advances in perinatal care have allowed for the survival of an increasingly larger population of neonates weighing less than 1000 g, referred to as extremely-low-birthweight (ELBW) infants. Because of limited supplies of endogenous energy reserves, the ELBW infant is extremely vulnerable to inadequate nutritional intake. The optimal strategy for supplying nutrition to these infants is poorly defined because of limited information on their substrate metabolism. Using common parenteral feeding regimens, we propose to study protein kinetics and the effects of the quality of infused energy on protein metabolism in the ELBW infant in the first 48 hours of life using stable isotope methodology combined with indirect calorimetry. Stable isotopic tracers of leucine, glucose and glycerol will be infused simultaneously to determine protein synthesis and oxidation, glucose production and utilization, and the rate of lipolysis. As part of this work, validation of a neonatal indirect calorimeter designed specifically for use in premature ventilated and non-ventilated infants will be completed. A neonatal activity monitor is also being developed to use as calorimetry for further refinement of metabolic data collection.